A review on recent approaches in the field of hot dip zinc galvanizing process

    A review on recent approaches in the field of hot dip zinc galvanizing process S.M.A. Shibli, B.N. Meena, R. Remya PII: DOI: Reference:

S0257-8972(14)01198-0 doi: 10.1016/j.surfcoat.2014.12.054 SCT 19991

To appear in:

Surface & Coatings Technology

Received date: Accepted date:

7 November 2014 22 December 2014

Please cite this article as: S.M.A. Shibli, B.N. Meena, R. Remya, A review on recent approaches in the field of hot dip zinc galvanizing process, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.12.054

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ACCEPTED MANUSCRIPT A review on recent approaches in the field of hot dip zinc galvanizing process

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S.M.A. Shibli*, B.N. Meena, R. Remya Department of Chemistry, University of Kerala, Kariavattom Campus

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Thiruvananthapuram, Kerala-695 581, India

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Abstract

The recent developments in the field of hot dip zinc coating are reviewed with special

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reference to different industrial applications. The improvements in physical and chemical structural composition due to pre and post modification process are discussed. The present review has the focus mainly on the readership of young researchers engaged in this field. Very recent developments on the hot dip galvanization processes are highlighted. Their

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industrial competencies with aluminium dipping are also briefly discussed. The scopes for

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immediate future developments are also highlighted then and there.

Key words: Hot dip galvanization, Intermetallics, Metal/metal oxide incorporation,

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Galvannealed coating, Aluminide coating.

---------------------------------------------------------------------------------------------------------*Author for all correspondence: [email protected] Phone: + 91 85470 67230 (mob), +91 471 2308682 (off), +91 471 2167 230 (Res)

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ACCEPTED MANUSCRIPT 1. Introduction Steel of different forms, is an integral part of building and construction industry due to its high strength and durability. Although many new and advanced materials have been

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developed for engineering applications, steel is still considered to be the main construction material for automobiles, appliances and industrial machinery [1]. Steel undergoes corrosion

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when exposed to different environments. There are different methods to prevent corrosion such as cathodic protection, anodic protection, addition of inhibitors, protective coatings and

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metallic coatings. Zinc coatings are extensively used for the protection of steel. In such cases, the more active zinc metal corrodes preferentially than the steel substrate by a cathodic reaction that prevents steel from undergoing anodic corrosion reaction. Different types of

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zinc coatings include: hot dip galvanizing (batch or continuous), electroplating, metalizing (zinc spraying), mechanical plating and zinc rich paint. Among them, the hot dip galvanization process, offers a unique combination of superior properties such as high strength, formability, light weight, corrosion resistance, low cost and recyclability. In a

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conventional hot dip galvanizing process, a steel article is cleaned, fluxed and then immersed

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in a molten zinc bath at a temperature of about 450 °C [2]. Hot dip galvanized steels have been extensively used in industrial fields such as automobiles, electrical home appliances or construction due to their excellent corrosion resistance characteristics [3]. This technique has

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been adopted as a well proven feasible process since 1800 soon after the exploration of iron and zinc. The exploration of the process was attempted in 1742 when a French Chemist Melouin presented a paper on hot dip galvanizing, and the process received commercial momentum with patents mainly in the 1830‟s. The reason for the extensive use of hot dip galvanization is the two-fold protective nature of the coating. As a barrier coating, it provides a tough, metallurgically bonded zinc coating that completely covers the steel surface and protects steel from corrosion. Additionally, the sacrificial action of zinc protects the steel even when damage or a minor discontinuity occurs on its surface. A hot dip galvanized coating consists of a heterogeneous assembly of different phases which are formed due to metallurgical reactions between iron and zinc when a ferrite substrate is immersed into molten zinc [4]. After solidification, the coating consists of an outer layer of 100 % zinc (η-eta layer) and inner layers called alloy layers consisting of intermetallic phases of iron and zinc such as zeta (ζ) layer (94 % Zn – 6 % Fe), delta (δ) layer (90 % Zn – 10 % Fe) and gamma (Г) layer (75 % Zn – 25 % Fe) [5-7] (Fig. 1). These intermetallic layers are relatively harder than the underlying steel and provide exceptional 2

ACCEPTED MANUSCRIPT protection against coating damage. The characteristics of the intermetallic phases of hot dip zinc coatings are compared in Table1 [8-11].

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2. The process

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2.1. Selection of substrate

The composition of a steel substrate has great influence on the hot dipping process and the performance of the resultant hot dip zinc coating. The change in the composition of

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the substrate not only influences the rate of attack of steel by molten zinc but also changes the mode of attack at a given galvanizing temperature [12]. The compositional variation of the

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steel used for hot dip zinc coating is an important factor that determines the zinc drainage, coating morphology and protection capacity of the coating. The chemical composition of the steel substrate also influences the metallurgical properties of the hot dip zinc coatings. The presence of Si, P, C and Mn in the steel substrate can influence the Fe-Zn solidification

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mechanism depending on their concentration. The presence of critical amount of silicon and

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phosphorus in the steel substrate is necessary to control the coating weight and the presence of carbon & phosphorus accelerates the growth of the alloy layer, thereby improving the adherence of the coating [13,14]. There are mainly two groups of steels that are used for hot

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dip galvanization namely low carbon, non-killed steel with low silicon and reactive steel with high silicon content.

2.2. Preparation of the base surface Surface preparation is an important step in hot dip galvanization because zinc does not metallurgically react with steel surface when it is not completely clean. The effective cleaning of steel substrate can be achieved by a variety of processes. Surface preparation typically consists of degreasing, pickling and fluxing. Oils, greases and other saponificable compounds present in the steel substrates are removed through degreasing. Alkaline solutions are normally used for this purpose since they are less expensive than vapour degreases using costly organic solvents. Yuttanant et al. [15] and R. Sa-nguanmo et al. [2] used to degrease the low carbon cold rolled steel substrate using 10 % NaOH solution at 60 °C for 10 minute prior to hot dip galvanization. The steel substrate could also be degreased using 5 % NaOH solution at 50 °C to ensure that the substrate is free from foreign materials [16].

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ACCEPTED MANUSCRIPT Normally steel substrate contains oxides of iron, even after the removal of greasy substances and that are to be removed by pickling. The warm/hot acid solutions such as HCl and H2SO4 are commonly used for the pickling process as both provide same pickling effect.

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But most of the researchers prefer HCl as pickling agent due to its use at low temperature, with less volume. Moreover, it is easy to inhibit and the steel substrate doesn‟t require any

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caustic treatment [17]. Shibli et al. have reported about the usage of 8 % HCl as a pickling agent for steel substrate [16]. It should be noted that steel coupons could also be pretreated by pickling in 14 % HCl at room temperature for 20 min [15, 2]. Stieglitz et al. have reported the

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use of a pickling solution of HCl and H3PO4 to ensure a stable coating [18]. Fluxing is required to dissolve any oxide films formed on the steel surface after

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pickling. It should be ensured that a clear metal surface contacts the molten zinc during the galvanization process. The fluxing treatment provides good adherence of liquid zinc on the steel substrate and facilitates adequate metallurgical interaction between zinc and steel. It also suppresses in-situ oxidation of the steel surface by atmospheric oxidation. Fluxing solutions

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consist of alkali and alkaline earth metal chlorides or fluorides with zinc chloride. The

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conventional fluxing solution consists of a mixture of zinc chloride (ZnCl2) and ammonium chloride (NH4Cl) in 1:3 mole ratio [19]. The presence of ammonium chloride in the flux promotes drying of the flux on the steel substrate before its entry into the molten zinc bath

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and zinc chloride protects the steel from corrosion or surface oxidation prior to its entry into the molten zinc bath. The flux coated steel substrate is dried at a temperature of about 120 °C since the flux decomposes at about 150 °C. A salt mixture containing zinc, ammonium, sodium, potassium, cobalt and lead chloride has been formulated based on the decomposition temperature of individual chloride salts for Zn – 5 wt. % Al alloy coating on wire surface [20]. Yuttanant et al. have reported that a fluxing solution that contains NiCl2 could affect the growth morphology of zeta phase and reduce its growth rate, resulting in the reduction of the overall thickness of the coating [15]. The use of NiCl2 in a fluxing solution serves as a simple and effective method for preventing excessive growth of galvanized coatings. Balloy et al. have reported the use of vegetable oil like linseed oil as fluxing agent rather than conventional industrial chloride flux [21]. The use of a mineral oil with an acid function such as HCl has also been reported as a fluxing agent during hot dip galvanization. However the usage of vegetable or mineral oils is yet to be proved commercially. There is no scope for commercialization of such processes as conventional fluxing does not involve with much cost. Similar is the case regarding the proposal by Balloy et al. who has proposed a process called “single bath” in order to merge the steps of pickling & fluxing. In this case the same 4

ACCEPTED MANUSCRIPT bath has HCl at the bottom to accelerate the pickling of steel while the mineral oil at the top is used as flux [21]. Such complicated processes have limited scope of reaching industry as

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many more recent developments have been emerged for this purpose.

2.3. Hot dip galvanization

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During hot dip galvanization, the steel substrate is completely immersed in a bath consisting of a minimum of 98 % pure molten zinc. The bath chemistry is specified by the

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ASTM in specification B6. The bath temperature is maintained at about 450 °C. The zinc metal then reacts with the iron on the steel surface to form a zinc/iron intermetallic alloy. The articles are withdrawn slowly from the galvanizing bath and the excess zinc is removed by

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draining, vibrating or centrifuging. The articles are cooled in air immediately after withdrawal from the bath. The characteristics of the resultant hot dip zinc coatings depend on the following parameters.

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2.3.1. Bath temperature

Conventionally the hot dip bath is maintained at around 440-460 °C. However, most of the significant chemical reactions between iron-zinc alloy coatings and the molten zinc

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occur at 480 °C. Below 480 °C, a compact Zn-Fe alloy is formed at the surface of the steel with the alloying action eventually ceasing up and above this critical temperature, fragmentation of the alloy layer is happened that stimulates zinc to penetrate into the metal resulting in the formation of very large amount of dross. The bath temperature can be controlled to tune the nature and thickness of the intermittent layers to suit the purpose of the specific application.

2.3.2. Dipping time The alloying reactions proceed after about 20 seconds of immersion of the substrate into the molten zinc bath. Conventionally 4-5 minutes duration is maintained for optimum alloying reactions. In certain cases where heavy articles are to be galvanized, they are kept immersed for longer durations to enhance zinc penetration as well as the overall coating thickness. The dipping duration can be controlled to tune the nature and thickness of the intermittent layers and also the extent of coverage.

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ACCEPTED MANUSCRIPT 2.3.3. Withdrawal speed The steel substrate is taken out of the bath when the reaction between iron and zinc completed. The thickness of hot dip galvanized coatings depends on withdrawal speed.

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Conventionally an optimum withdrawal rate of 1.5m per minute is maintained for the formation of a bright shiny zinc coating. If the withdrawal speed is too slow, a uniform

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unalloyed zinc layer is formed while in the case of faster withdrawal speed, an uneven coating is formed. Hence, the withdrawal speed should be controlled to tune the nature,

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thickness and the surface finishing of the coating.

2.3.4. Bath composition

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The extent of effective life of hot dip zinc coating not only depends on the coating composition but also on the metallurgical characteristics of the coating. It is necessary to increase the quality and thus the service life of the coating by making improvements in the galvanizing techniques. Several additives are incorporated into molten zinc bath in order to

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improve the performance of hot dip zinc coatings. The presence of elements such as

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aluminium, magnesium, and nickel plays an important role in improving the galvanic performance of hot dip zinc coatings. Aluminium is one of the commonly used additives in molten zinc bath. Aluminium has the ability to reduce the rate of oxidation of molten zinc

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and to reduce the spangle size thereby improves the uniformity of the coating. The effect of aluminium on the bath has also been extensively studied for improving the corrosion resistance [22,23]. During hot dip galvanization, the presence of aluminium in the molten zinc bath suppresses the growth of Fe-Zn intermetallic layer and reduces iron loss into the bath through the formation of Fe2Al5 and FeAl3 intermetallic layers. It has been reported that the presence of Al (0.15 – 0.2 wt. %) in molten zinc bath can prevent the growth of Fe-Zn intermetallic layer at the steel coating interface [24]. Liberski et al. has reported that hot dip zinc coating containing 5 wt. % Al could possess excellent corrosion resistance than conventional zinc coatings [25]. The most commonly used industrial hot dip zinc coatings with aluminium as additive are the galvanized iron (Zn – 0.2 wt. % Al), galvalume (55 wt. % Al, 43.5 wt. % Zn, 1.5 wt. % Si) and galfan (Zn – 5 wt. % Al) [26]. The addition of lead and antimony in small concentrations (0.004 – 0.200 %) could significantly improve the uniformity and adherence of the coating [27]. Pistofidis et al. have found that the addition of bismuth into the galvanizing bath could yield excellent adhesion and corrosion resistance

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ACCEPTED MANUSCRIPT [28]. However the compositional modification using elements such as lead and antimony is seemed to have no scope for receiving any commercial attention.

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3. Evaluation of coating composition The important properties that concern the use of zinc coatings are primarily corrosion

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and formability and other properties involved are weldability & paintability. The progress of corrosion in different types of galvanized coatings can be investigated by using Electrochemical Impedance Spectroscopic (EIS) technique. V. Barranco et al. have compared

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the corrosion rate of hot dip galvanized coatings especially pure zinc, Zn – 5 wt. % Al and Zn – 10 wt. % Fe in 3 % NaCl, based on EIS measurements. The EIS results revealed that

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corrosion of the pure zinc & Zn – 5 wt. % Al coatings progresses in an almost uniform way while the corrosion rate of the Zn – 10 wt. % Fe coating experiences a decrease after 10 days of exposure [29]. J.H. Hong et al. have reported that the Mossbauer properties can be used for the phase formation and transformation studies as a function of the process parameters. The

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Mossbauer spectra of different Fe-Zn intermetallic phases are different from each other.

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According to them, the gamma and gamma-1 phases are composed of four sub spectra, while the delta and zeta phases reveal three and one sub spectrum respectively [30]. H. Liu et al. have studied the influence of H2 and water vapour content on selective oxidation during

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continuous hot dip galvanization by thermo-calc and Wagner model. As per the model, the nature of oxidation process (internal/external) can be changed by altering H2 content in the annealing atmosphere. A study based on thermodynamic characteristics of the process has revealed that simple oxides such as MnO2, Cr2O3 etc produced by selective oxidation may be reduced by effective Al in Zn liquid [31]. Thus, in-situ and post evaluation of the coating characteristics could yield significant information to alter the composition of the bath and the coating.

4. The phase reaction and formation of intermetallics Dutta et al. have reported about the formation of MgZn2 on the top surface and described about the morphology and properties of Zn-Mg & Zn-Mg-Al coatings [32]. They also discussed about the Mg gradient in the coating. Liu et al. have studied the relationship between the annealing atmosphere and microstructure of Zn-Al coated dual phase steel. They discussed about the coexistence of Fe-Al intermetallic compounds and needle like Fe-Zn 7

ACCEPTED MANUSCRIPT compound as the result of reduction in aluminothermic reaction during hot-dip zinc process [33]. Song et al. have found that zinc coating on dual phase steel consists of a zinc layer and columnar –FeZn13 particles on the top of a thin inhibition layer next to steel substrate. These

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inhibition layers consist of lenticular –Fe2Al5-xZnx particles [34]. Yang et al. have studied the influence of strip entry temperature on the formation of interfacial layer during hot dip

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galvanization of hardened steel. He has illustrated that the size of -phase was increased with strip entry temperature and was formed at the base steel surface when temperature was 480 °C. According to Yang et al. the Fe-Al intermetallic layer was fully formed at strip entry

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temperature ranging from 440-480 °C [35].

Cheng et al. reported the formation of a new quaternary phase in the interdentritic

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area of a laser treated Zn-Al-Mg-Si coating [36]. Yang et al. studied the growth of 2 intermetallic compounds on hot dipped Zn-Ti coating. The 2 particles generated in the η layer prevents the generation of compact  layer in Zn – 0.05 % Ti coating. According to them, higher Ti content increases the number of precipitated particles [37]. Peng et al.

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reported that a typical shiny, feathery and dull spangle can be obtained by batch hot dip

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galvanizing in Zn – 0.05 Al – 0.20 Sb bath. A large number of precipitated -Sb3Zn4 particles get distributed randomly on the shiny spangle surface. Both -Sb3Zn4 particles and the dentritic segregation of Sb result in spacing of feathery spangles on the surface [38]. The bath

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parameters including the elemental and other additives such composites should be controlled to tune the nature and thickness of the intermittent layers to suit the purpose of the specific application of the coating.

5. Metal and metal oxide addition in molten zinc bath Mg and Mg-Al additions in the galvanizing bath are a proven technique to regulate the intermittent reactions improving the coating morphology and properties of low carbon steel. The hardness of Zn-Mg & Zn-Mg-Al coatings is normally higher than that of pure zinc coating, a favorable character for scratch resistance. The Zn - 0.5 wt. % Mg – 0.25 wt. % Al coating has more corrosion resistance than pure zinc & Zn-Mg coatings [32]. Duchoslav et al. have studied the initial progress of atmospheric corrosion of tenancy alloyed Zn-Mg-Al coatings in aqueous NaCl environment. They have also reported that two fundamental processes are involved in the initial corrosion attack. This results in dissolution of the surface layer consisting of MgO and preferential anodic dissolution of both binary and ternary eutectic phases constituting the coating [39]. Kondratiuk et al. have reported about the 8

ACCEPTED MANUSCRIPT formation of a ZnO layer on Zn-Ni coating during heat treatment. The electroplated uniphase intermetallic Zn-Ni coatings are highly suitable for hot sheet metal forming applications owing to their good thermal stability compared to hot dip zinc coatings [40]. The presence of

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Mg in zinc coatings causes a remarkable improvement in the corrosion resistance of painted systems. Hausbrand et al. have studied the corrosion of painted MgZn2 with a defect, under

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constant conditions of high humidity and an electrolyte covered defect [41]. They discussed about the galvanic coupling among the defects and also about the galvanic coupling between defect and intact interface. According to them, no cathodic delamination is possible due to an

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unfavorable potential gradient between the defect and the intact interface in the case of MgZn2. An anodic delamination is possible through the migration of ions at the metal oxide

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interface [41]. LeBozec et al. have compared the corrosion performance of Zn-Mg-Al coatings with conventional zinc coatings such as hot dip galvanized, electrogalvanized, Galvannealed and galfan coatings. The performance of ZnMgAl coatings is depending on the testing conditions and the configuration of the samples and these coatings provide a major

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improvement during neutral salt spray analysis [42].

High performance hot dip zinc coatings can be developed by the incorporation of individual and mixed metal oxides such as nano TiO2, CeO2-TiO2 and Al2O3-ZrO2 into the

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hot dip zinc coating [43-45]. The incorporation of Al2O3-ZrO2 mixed oxide composite into galvanizing bath yields aluminium rich zinc coatings with high sliding and wear resistance. The interior layers of the coatings are also known to possess high stability against corrosion. The incorporation of nano TiO2 in the hot dip zinc coating has been known to facilitate phosphating and also good paintability. The hot dip galvanized coatings with high corrosion resistance, effective barrier protection, good antifouling characteristics and improved surface quality can be achieved by CeO2-TiO2 incorporation. These composites not only yield individual characteristics to the coating but also give a new range of reaction modification during the process.

6. Annealing of hot dip zinc coatings The annealing of hot dip galvanized coatings (galvannealed) leads to the formation of Fe-Zn intermetallic phases. Galvannealed coatings differ from pure zinc coatings that they consist of a layered structure of different Fe-Zn intermetallic phases and have superior 9

ACCEPTED MANUSCRIPT resistance to corrosion, improved paintability and better weldability than pure zinc coatings [46]. Chakraborty et al. have studied the root cause of hitherto unknown and uncommon defect that has been encountered in the coating of an industrial galvannealed HIF steel. The

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root cause of the defect lies in the presence of some pickling resistant sticky iron oxide on the surface of the hot rolled sheet, which exists even after cold rolling. According to them, the

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subsequent galvannealing operation has no effect on Fe-Zn reaction at the spots covered by the oxide layer and it causes defects in the final product [47]. H. Y. Ha et al. examined the dissolution process of a galvannealed coating layer on a dual phase steel and the variation in

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the corrosion rate during the stripping process. According to them, the corrosion rate gets accelerated when the outermost  phase is completely dissolved and the galvanic couple of

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the δ-Г substrate is exposed to the 1M NaCl + 0.01M H2SO4. However uniform and thick layer of  phase is required for enhanced corrosion prevention [48]. Kim et al. have evaluated the mechanical properties of galvannealed steel sheets used for automotive exposed panel and the failure phenomenon of its coating layer using finite element method. They have reported

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that the failure of the coating layer occurs when tensile deformation mode is activated at the

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coating layer and equivalent local plastic strain becomes more than 0.28 [49]. Okamoto et al. have studied the compression deformability of  and  Fe-Zn intermetallics to mitigate detachment of brittle intermetallic coating of galvannealed steels.

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The  phase is the most deformable followed by the  phase, while the 1, 1k and 1P phases are all highly brittle based on compression tests of micrometer sized single phase specimens [50]. Manna & Dutta have reported that the charge transfer resistivity of galvannealed coatings with different intermediate layers is better than similar galvanized coatings and also reported that the galvanized and galvannealed coatings with prior metal flash coatings show better mechanical performance compared to the coatings obtained without prior metal flash coating [51]. Choi et al. have studied the effect of pre-electroplating with prior to reduction of annealing on the surface quality and resultant corrosion characteristics of galvannealed high strength DP steel. The degree of Fe-Zn alloying can be significantly improved due to the formation of a homogeneous interfacial Fe2Al5 layer at the coating/substrate interface during the galvanizing process, resulting in an increase in the thickness of the coating layer. These changes arise due to the suppression of the segregation and selective oxidation of small amount of alloying elements during reduction in the annealing process prior to the galvanization [52]. Thus, apart from the bath reactions which are mainly controlled by the composition and the dipping conditions, some superficial reactions can be tuned by 10

ACCEPTED MANUSCRIPT secondary parameters such as energy and strain during the annealing process. Such controlled and post treated coatings only find extensive industrial applications.

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7. Recent developments in hot dip galvanization of high strength steel The performance of automotive designs requires high strength materials with good

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formability, fatigue resistance and toughness [53]. The use of advanced high strength steels in automotive industry has saved weight and enables economy of fuel. The protection of high

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strength steel against corrosion by hot dip galvanization became an important issue in recent years because it is an integral part of automotive industry. The developments in hot dip galvanized high strength steels have been reported extensively. Some of the recent catches

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are discussed in this review. The high strength steels undergo recrystallization annealing prior to hot dip galvanization. The annealing in the reducing atmosphere causes segregation and selective oxidation of alloying elements, making the steel surface unsuitable for galvanization. The wetting nature of zinc is decreased by the presence of external oxides of

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elements such as Al, Si and Mn on the surface of steel [54]. Frenznick et al. have reported

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about the influence of Si, Mn oxides and their surface coverage on wetting kinetics of zinc coatings. According to them, the Cassie equation along with Avrami growth law was a good approximation for the wetting kinetics and the growth mechanism of reaction layer. They

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found the reactive wetting dynamics depends not only on the overall oxide coverage but also on the size of the oxide islands [55]. H. Liu et al. (2012) have reported about the surface selective oxidation of alloying elements and mechanical property degradation of the hot dip galvanized high strength dual phase steel by comparing three dual phase steels such as Mn-Si steel, Cr steel and Cr-Mo steel. According to them, the surface segregation and the selective oxidation are greatly influenced by the dew point of annealing atmosphere and the steel composition [56]. Blumenau et al. have studied the effect of pre oxidation on improving the reactive wetting of high manganese alloyed steel during hot dip galvanization. It becomes clear that the pre oxidation offers several advantages regarding technical feasibility [57].

8. Competency with aluminium dipping 8.1. The advantages of aluminium hot dipping Aluminium and aluminium alloy coatings are used in automobile and construction industries as an alternative for zinc coating in order to reduce weight, provide better 11

ACCEPTED MANUSCRIPT mechanical properties and yield high temperature oxidation resistance compared to hot dip zinc coating. The corrosion resistant characteristics of aluminized steel are due to the formation of a stable thin film of aluminium oxide. Hot dip aluminizing is a diffusion coating

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formation process used to deposit high temperature oxidation and corrosion resistant coatings on stainless steels and low alloy steels [58-60]. The dipping of steel in molten aluminium

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causes interdiffusion of the aluminium and iron leads to the formation of different intermetallics [61]. The coating on hot dipped aluminide steel is composed of an aluminium top coat and a thick (10-100 m) brittle Fe-Al intermetallic layer. The Fe-Al intermetallic

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layer comprises an outer FeAl3 layer (often called Fe4Al13 [61]) and an inner Fe2Al5 layer [62]. The Fe2Al5 layer determines the development of coating phases and the final coating

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properties [63]. Hot dipped aluminide mild steels have been widely used in boilers and exhaust pipes in high temperature environments, due to the formation of fine, dense Al2O3 with good oxidation resistance on the surface of steel [64]. In order to improve the lifetime of hot dipped aluminide steel in high temperature conditions, it is essential to create a diffusion

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barrier between the steel substrate and the aluminide coating.

8.2. Influence of alloying elements in the formation of intermetallic layers vs aluminium Cheng & Wang have studied the effect of silicon addition in the Al bath on the

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oxidation behaviour of hot dipped aluminide mild steel [65]. Aluminide steel with high Si having poor isothermal oxidation resistance is attributed to its thin aluminide layer and formation of a large number of phase transformation induced voids. Aluminide steel with a thick Fe2Al5 layer in the low Si content aluminide layer has low cyclic oxidation resistance because the low fracture toughness of Fe2Al5 phase enhances crack formation in the aluminide layer [65]. Windmann et al. have reported about the transformation of brittle Al-Si coatings into more ductile phase of type Al-Fe during austenitization and have studied the phase formation as a function of coating thickness and Si content [66]. The presence of silicon influences the diffusivity of Al & Fe in the Al-Fe rich intermetallics and promotes the formation of Si rich intermetallics of type Al8Fe2Si, Al13Fe4 and Al5Fe2. The transformation of Al rich intermetallic phases of type Al5Fe2 & Al13Fe4 into iron rich phase of type AlFe can be enhanced by higher Si content (10 mass%) and by decreasing the coating thickness [66]. Cheng et al. have reported about the factors affecting the thickness and morphology of the Al-Si coated CLAM steel. Fingerlike intermetallic layers are formed under optimum conditions of 1073 K, 3 minute and Al – 5.8 at. % Si. The existence of certain amount of Si in 12

ACCEPTED MANUSCRIPT the molten Al bath suppresses the growth of Fe2Al5, builds up the thickness of FeAl3 slightly that causes reduction in the thickness of intermetallic layer [67]. Later Danzo et al. have reported the microstructural and crystallographic features of steel during hot dipping with Al-

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Si bath followed by diffusion annealing treatment (900-1200 °C for 1 hour). Columnar grain shape can be achieved during diffusion annealing and the intermetallic layers formed during

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hot dipping can be vanished, as they serve as an Al source for columnar grain growth [68]. Takata et al. have also studied the morphology and growth of Fe-Al intermetallic layers formed on pure Fe sheets dipped in Al – 8.2 Mg – 4.8 Si alloy melt at 750 °C. The

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intermetallic layer exhibits a dual layer structure which consists of a continuous -FeAl3 and a large η-Fe2Al5 phase layers. The presence of Si and Mg prevents the diffusion of Fe into Al

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melt and thereby the growth of η phase layer and promotes the growth of  phase layer. The  phase acts as the diffusion barrier in Al-Si-Mg aluminide coating [69]. Cheng et al. have found that a thin intermetallic layer composed of (Fe-Ni)2 Al9, NiAl3 and Ni2Al3 and a thick intermetallic layer composed of FeAl3 and Fe2Al5 are formed in the aluminide/nickel duplex

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coating on mild steel. In such case, nickel pre-plating has no effect on the growth rates of

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FeAl3 & Fe2Al5 as it only slows down the initial growth rate for FeAl3 and Fe2Al5 which can be attributed to the presence of Ni2Al3, NiAl3 and (Fe, Ni)2Al9 in the intermetallic layers before the formation of FeAl3 & Fe2Al5. As the immersion time is increased, nickel pre-

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plating layer is fully consumed due to its dissolution into the Al bath resulting in intermetallic layers contained only outer minor FeAl3 and inner major Fe2Al5. Among all the phases formed, Fe2Al5 had the fastest growth rate [62]. 8.3. Corrosion resistant and oxidation resistant characteristics of aluminide coating vs aluminium

Frutos et al. have reported the oxidation behaviour of hot dipped AISI 316 LVM stainless steel in molten Al - 31at. % Si bath. At 900 °C, the rapid transformation of less protective aluminas into the protective –Al2O3 enhances the oxidation resistance of the aluminized material compared with uncoated one. Below 900 °C, the chromia layer is more protective than the scale consisting of less protective aluminas. As a result the coating had no beneficial effect on the oxidation resistance of stainless steel [70]. Lemmens et al. have compared the electrochemical behavior of hot dipped DC06 European grade steel in Al & AlSi bath. According to them the crates do not corrode while the outer Al layer can dissolve sacrificially to protect all underlying layers and the steel substrate in chloride environments 13

ACCEPTED MANUSCRIPT [61]. According to Cheng et al., Ni-Al alloys possess better corrosion resistance than Fe-Al alloy in chloride containing atmospheres. Therefore, Ni-aluminide coating has been widely used to improve the hot corrosion resistance of the substrate material [62]. Shi et al. have

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studied the effect of dipping temperature and heat treatment on the aluminide coatings characteristics. The thickness of the intermetallic layer increases with increasing dipping

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temperature. The transformation of multiphase structures of aluminizing layer into a single phase (Fe3Al) through heat treatment favors improvement on chemical stability, toughness, corrosion and wear resistance of T91 steel [71]. Ni et al. have studied the corrosion resistant

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characteristics of aluminium coating with and without annealing against molten carbonate using electrochemical impedance spectroscopy. These annealed coatings generally exhibit

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better corrosion resistance than Al-Fe intermetallic coating formed in-situ during the corrosion process. The main cause for the degradation of aluminide coatings is not only due to the corrosion of the coating in contact with molten carbonate but also due to the aluminium

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depletion through the interdiffusion of aluminium and the substrate [72].

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9. Conclusions

The role of composition of the substrate, the bath as well as the process parameters including annealing conditions significantly tune the structural characteristics and

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applicability of the coatings. Since the hot dipping process is an alloying reaction, the process conditions significantly extent/suppress/delaying the targeted reaction. Thus the process parameters of the whole processes including pretreatment, dipping and curing/annealing have a greater role than the role of the composition of the bath or other solutions involved in the process. The significance of presence of optimum amount of silicon and phosphorus in the steel substrate has been found to be crucial during the hot dipping process. Apart from the bath composition, the pretreatment and the dipping conditions alter the resultant coating to a great extent. Tuning of intermetallics and multiphase interdentrices can fulfill the formation of targeted characteristics of the hot dip coatings. Control of other in-situ physicochemical reactions such as control of segregation, selective oxidation of doping elements and suppression of selective intermetallic reactions are considered to be crucial during the hot dipping process.

14

ACCEPTED MANUSCRIPT Acknowledgement The authors thank the Head of the Department of Chemistry, University of Kerala for

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extending support to carry out the research work.

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References

[1] A. Amadeh, B. Pahlevani, S. Heshmati-Manesh, Corros. Sci. 44 (2002) 2321. [2] R. Sa-nguanmoo, E. Nisaratanaporn, Y. Boonyongmaneerat, Corros. Sci. 53 (2011)

[3] S. Maeda, Prog. Org. Coat. 28 (1996) 227.

US

122.

MA N

[4] G. Reumont, J.B. Vogt, A. Iost, J. Foet, Surf. Coat. Technol. 139 (2000) 265. [5] P.R. Sere, J.D. Culcai, C.I. Elsner, A.R.D. Sarli, Rev. Metal. Madrid. 33 (1997) 376. [6] C.S. Lin, M. Meshi, Metall. Mater. Trans. B. 25B (1994) 721. [7] S. Felin Jr, V. Barranco, Acta Mater. 51 (2003) 5413.

D

[8] P. Rajak, U. Tewary, S. Das, B. Bhattacharya, N. Chakraborti, Comput. Mater. Sci. 50

PT E

(2011) 2502.

[9] C.H.E. Belin, R.C.H. Belin, J. Solid State Chem. 151 (2000) 85. [10] W.H. Zhu, H.M. Jin, P. Wu, H.L. Liu, Phys. Rev. B 70 (2004) 165419. [11] A.S. Koster, J.C. Schoone, Acta Crystallogr. B 37 (1981) 1905.

AC CE

[12] D. Horstmann, „The influence of impurities in iron on attack by molten zinc‟, 4th Int. Galv. Conf. Milan (1956). [13] C.E. Jordan, A.R. Marder, J. Mater. Sci. 32 (1997) 5603. [14] M.S. Kozdras, P. Niessen, Mater. Sci. Technol. 6 (1990) 681. [15] Y. Boonyongmaneerat, K. Saengkiettiyut, P. Rattanawaleedirojn, C. Angkaprasert, J. Wanichsampan, S. Saenapitak, J. Iron Steel Res. Int. 17(8) (2011) 74. [16] S.M.A. Shibli, R. Manu, Surf. Coat. Technol. 197 (2005) 103. [17] T.H. Cook, Met. Finish. 89 (6) (1991) 107. [18] U. Stieglitz, M.W.D. Schulz, H. Kulker, V. Brucken, Metalloberflaeche 56 (2002) 11. [19] R.P. Krepski, Met. Finish. 87 (10) (1989) 37. [20] M. Manna, Surf. Coat. Technol. 205 (2011) 3716. [21] D. Balloy, J.Y. Dauphin, J.C. Tissier, Surf. Coat. Technol. 202 (2007) 479. [22] L. Allegra, J.C. Zoccola, Mater. Performance 22 (1983) 5. [23] S.M.A. Shibli, A.C. Jayalekshmi, R. Remya, Surf. Coat. Technol. 201 (2007) 7560. [24] S. Maeda, Prog. Org. Coat. 28 (1996) 227. 15

ACCEPTED MANUSCRIPT [25] P. Liberski, K. Henryk, P. Podolski, A. Gierek, Ochrona przed Korozja, 47 (2004) 264. [26] A. R. Marder, Prog. Mater. Sci. 45 (2000) 191. [27] P.R. Sere, J.D. Culcasi, C.I. Elsner, A.R. Di Sarli, Surf. Coat. Technol. 122 (1999) 143.

PT

[28] N. Pistofidis, G. Yourlias, S. Konidaris, E. Pavilidou, A. Stergiou, G. Stergioudis, Mater. Lett. 61 (2007) 994.

CR I

[29] V. Barranco, S. Feliu Jr., S. Feliu, Corros. Sci. 46 (2004) 2203. [30] J.H. Hong, S.J. Oh, S.J. Kwon, Intermetallics 11 (2003) 207. [31] H. Liu, Y. He, L. Li, Appl. Surf. Sci. 256 (2009) 1399.

US

[32] M. Dutta, A.K. Halder, S.B. Singh, Surf. Coat. Technol. 205 (2010) 2578. [33] H. Liu, F. Li, W. Shi, R. Liu, L. Li, Surf. Coat. Technol. 205 (2011) 3535.

MA N

[34] G.M. Song, T. Vystavel, N. van der Pers, J.Th.M. De Hosson, W.G. Sloof, Acta Mater. 60 (2012) 2973.

[35] H. Yang, S. Zhang, J. Li, X. Liu, H. Wang, Surf. Coat. Technol. 240 (2014) 269. [36] Z. Chen, C.T. Peng, Q. Liu, R. Smith, D. Nolan, J. Alloys Compd. 589 (2014) 226. Q. Xu, Y. Guo, J. Iron Steel Res. Int. 21(3) (2014) 396.

D

[37] Y. Gui,

PT E

[38] S. Peng, J. Lu, C. Che, G. Kong, Q. Xu, Appl. Surf. Sci. 256 (2010) 5015. [39] J. Duchoslav, M. Arndt, R. Steinberger, T. Keppert, G. Luckeneder, K.H. Stellnberger, J. Hagler, C.K. Riener, G. Angeli, D. Stifter, Corros. Sci. 83 (2014) 327.

AC CE

[40] J. Kondratiuk, P. Kuhn, E. Labrenz, C. Bischoff, Surf. Coat. Technol. 205 (2011) 4141. [41] R. Hausbrand, M. Stratmann, M. Rohwerder, Corros. Sci. 51 (2009) 2107. [42] N. LeBozec, D. Thierry, A. Peltola, L. Luxem, G. Luckeneder, G. Marchiaro, M. Rohwerder, Mater. Corros. 64 (2013) 969. [43] S.M.A. Shibli, F. Chacko, C. Divya, Corros. Sci. 52 (2010) 518. [44] S.M.A. Shibli, F. Chacko, Appl. Surf. Sci. 257 (2011) 3111. [45] S.M.A. Shibli, F. Chacko, Surf. Coat. Technol. 205 (2011) 2931. [46] C. Xhoffer, H. Dillen, B.C. Cooman, J. Appl. Electrochem. 29 (1999) 209. [47] A. Chakraborty , M. Dutta , R. Pais , R.K. Ray, Surf. Coat. Technol. 204 (2010) 3481. [48] H.Y. Ha, S.J. Park, J.Y. Kang, H.D. Kim, M.B. Moon, Corros. Sci. 53 (2011) 2430. [49] S.I. Kim, J.U. Her, Y.C. Jang, Y. Lee, Trans. Nonferrous Met. Soc. China 21 (2011) s111. [50] N.L. Okamoto, D. Kashioka, M. Inomoto, H. Inui, H. Takebayashi, S. Yamaguchid Scripta Mater. 69 (2013) 307. [51] M. Manna, M. Dutta, Surf. Coat. Technol. 251 (2014) 29. [52] Y.I. Choi, E.S. Shin, K. Kuroda, M. Okido, C.J. Park, Corros. Sci. 58 (2012) 152. 16

ACCEPTED MANUSCRIPT [53] J. Sun, H. Yu, Mat. Sci. Eng. A 586 (2013) 100. [54] H.J. Grabke, V. Leroy, H. Viefhaus, ISIJ Int. 35 (1995) 95. [55] S. Frenznick, S. Swaminathan, M. Stratmann, M. Rohwerder, J. Mater. Sci. 45 (2010)

PT

2106. [56] H. Liu, F. Li, W. Shi, S. Swaminathan, Y. He, M. Rohwerder, L. Li, Surf. Coat.Technol.

CR I

206 (2012) 3428.

[57] M. Blumenau, M. Norden, F. Friedel, K. Peters, Surf. Coat. Technol. 206 (2011) 559. [58] C.J. Wang, S.M. Chen, Surf. Coat. Technol. 201 (2006) 3862.

US

[59] C.C. Tsaur, J.C. Rock, Y.Y. Chang, Mater. Chem. Phys. 91 (2005) 330. [60] C.J. Wang, C.C. Li, Surf. Coat. Technol. 177–178 (2003) 37.

MA N

[61] B. Lemmens, Y.G. Garcia, B. Corlud, J.D. Strycker, I.D. Graeve, K. Verbeken, Surf. Coat. Technol. (2014) Article in Press, DOI: 10.1016/j.surfcoat.2014.06.064. [62] W.J. Cheng, C.J. Wang, Mater. Charact. 69 (2012) 63. [63] M. Dutta, S.B. Singh, Scripta Mater. 60 (2009) 643.

D

[64] W.J. Cheng, C.J. Wang, Surf. Coat. Technol. 204 (2009) 824.

PT E

[65] W.J. Cheng, C.J. Wang, Appl. Surf. Sci. 274 (2013) 258. [66] M. Windmann, A. Röttger, W. Theisen, Surf. Coat. Technol. 246 (2014) 17. [67] X. Chen , Q. Huang, Z. Yan, Y. Song, S. Liu, Z. Jiang, J. Nucl. Mater. 442 (2013) S597.

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[68] I.I. Danzo, Y. Houbaert, K. Verbeken, Surf. Coat. Technol. 251 (2014) 15. [69] N. Takata, M. Nishimoto, S. Kobayashi, M. Takeyama, Intermetallics 54 (2014) 136. [70] E. Frutos, P. Adeva, J.L. González-Carrasco, P. Pérez, Surf. Coat. Technol. 236 (2013) 188.

[71] Z. Shi, J. Cao, F. Han, J. Nucl. Mater. 447 (2014) 77. [72] C.S. Ni, L.Y. Lu, C.L. Zeng, Y. Niu, J. Power Sources 261 (2014) 162.

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Г phase

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Table 1: Characteristics of Fe-Zn intermetallic phases of hot dip zinc coatings [8-11]

Fe5Zn21

Fe3Zn10

7-11.5

17-19.5

23.5-28

FCC

BCC

408

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ζ phase

δ phase

Stoichiometry

Zn

FeZn13

FeZn10

Wt % of iron

0

5-6

Crystal structure

HCP

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6

Monoclinic Hexagonal

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Atoms/unit cell

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η phase

555

Г1 phase

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Source: Comput. Mater. Sci., Vol. 50, 2011, pp. 2502 (Elsevier)

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eta

zeta delta gamma

steel

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Fig. 1. The intermetallic layers present in a typical / conventional hot dip galvanized

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coating.

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Highlights

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highlighted.

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iscussed.